bisect: avoid copying ancestor list for non-merge commits
During a bisection, hg needs to compute a list of all ancestors for every
candidate commit. This is accomplished via a bottom-up traversal of the set of
candidates, during which each revision's ancestor list is populated using the
ancestor list of its parent(s). Previously, this involved copying the entire
list, which could be very long in if the bisection range was large.
To help improve this, we can observe that each candidate commit is visited
exactly once, at which point its ancestor list is copied into its children's
lists and then dropped. In the case of non-merge commits, a commit's ancestor
list consists exactly of its parent's list plus itself. This means that we can
trivially reuse the parent's existing list for one of its non-merge children,
which avoids copying entirely if that commit is the parent's only child. This
makes bisections over linear ranges of commits much faster.
During some informal testing in the large publicly-available `mozilla-central`
repository, this noticeably sped up bisections over large ranges of history:
Setup:
$ cd mozilla-central
$ hg bisect --reset
$ hg bisect --good 0
$ hg log -r tip -T '{rev}\n'
628417
Test:
$ time hg bisect --bad tip --noupdate
Before:
real 3m35.927s
user 3m35.553s
sys 0m0.319s
After:
real 1m41.142s
user 1m40.810s
sys 0m0.285s
$ hg init
Revision 0:
$ echo "unchanged" > unchanged
$ echo "remove me" > remove
$ echo "copy me" > copy
$ echo "move me" > move
$ for i in 1 2 3 4 5 6 7 8 9; do
> echo "merge ok $i" >> zzz1_merge_ok
> done
$ echo "merge bad" > zzz2_merge_bad
$ hg ci -Am "revision 0"
adding copy
adding move
adding remove
adding unchanged
adding zzz1_merge_ok
adding zzz2_merge_bad
Revision 1:
$ hg rm remove
$ hg mv move moved
$ hg cp copy copied
$ echo "added" > added
$ hg add added
$ echo "new first line" > zzz1_merge_ok
$ hg cat zzz1_merge_ok >> zzz1_merge_ok
$ echo "new last line" >> zzz2_merge_bad
$ hg ci -m "revision 1"
Local changes to revision 0:
$ hg co 0
4 files updated, 0 files merged, 3 files removed, 0 files unresolved
$ echo "new last line" >> zzz1_merge_ok
$ echo "another last line" >> zzz2_merge_bad
$ hg diff --nodates | grep "^[+-][^<>]"
--- a/zzz1_merge_ok
+++ b/zzz1_merge_ok
+new last line
--- a/zzz2_merge_bad
+++ b/zzz2_merge_bad
+another last line
$ hg st
M zzz1_merge_ok
M zzz2_merge_bad
Local merge with bad merge tool:
$ HGMERGE=false hg co
merging zzz1_merge_ok
merging zzz2_merge_bad
merging zzz2_merge_bad failed!
3 files updated, 1 files merged, 2 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
[1]
$ hg resolve -m
(no more unresolved files)
$ hg co 0
merging zzz1_merge_ok
merging zzz2_merge_bad
warning: conflicts while merging zzz2_merge_bad! (edit, then use 'hg resolve --mark')
2 files updated, 1 files merged, 3 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
[1]
$ hg diff --nodates | grep "^[+-][^<>]"
--- a/zzz1_merge_ok
+++ b/zzz1_merge_ok
+new last line
--- a/zzz2_merge_bad
+++ b/zzz2_merge_bad
+another last line
+=======
$ hg st
M zzz1_merge_ok
M zzz2_merge_bad
? zzz2_merge_bad.orig
Local merge with conflicts:
$ hg resolve -m
(no more unresolved files)
$ hg co
merging zzz1_merge_ok
merging zzz2_merge_bad
warning: conflicts while merging zzz2_merge_bad! (edit, then use 'hg resolve --mark')
3 files updated, 1 files merged, 2 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
[1]
$ hg resolve -m
(no more unresolved files)
$ hg co 0 --config 'ui.origbackuppath=.hg/origbackups'
merging zzz1_merge_ok
merging zzz2_merge_bad
warning: conflicts while merging zzz2_merge_bad! (edit, then use 'hg resolve --mark')
2 files updated, 1 files merged, 3 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
[1]
Are orig files from the last commit where we want them?
$ ls .hg/origbackups
zzz2_merge_bad
$ hg diff --nodates | grep "^[+-][^<>]"
--- a/zzz1_merge_ok
+++ b/zzz1_merge_ok
+new last line
--- a/zzz2_merge_bad
+++ b/zzz2_merge_bad
+another last line
+=======
+=======
+new last line
+=======
$ hg st
M zzz1_merge_ok
M zzz2_merge_bad
? zzz2_merge_bad.orig
Local merge without conflicts:
$ hg revert zzz2_merge_bad
$ hg resolve -m
(no more unresolved files)
$ hg co
merging zzz1_merge_ok
4 files updated, 1 files merged, 2 files removed, 0 files unresolved
$ hg diff --nodates | grep "^[+-][^<>]"
--- a/zzz1_merge_ok
+++ b/zzz1_merge_ok
+new last line
$ hg st
M zzz1_merge_ok
? zzz2_merge_bad.orig